Add rate-limiting on 429 Too Many Requests errors
Use cl-flet in `pinboard-string-trim' definition

Avoids defining aliases for `string-trim-left' and `string-trim-right'
functions that are otherwise unused.
Fix autoloads
WIP: Add pinboard-bookmark-jump, pinboard-bookmark-list-filtered
Use url-http-response-status variable to detect 429s
Improve bookmark-fetching logic
WIP: Improve use of (interactive) specs

The idea here is to make the same functions usable interactively and for
scripting where possible.
document pinboard-list-tags
better interactive spec for pinboard-tag-follow
more docstrings
better pinboard-bookmark-jump